Turkish President Tayyip Erdogan said on Saturday a military operation in Syria’s Kurdish-controlled Afrin region had begun after cross-border shelling by the Turkish Army.

Turkey’s Army said on Saturday that it hit shelters and hideouts used by militants from three groups – the Kurdistan Workers’ Party (PKK), PYD and YPG, after militants fired on Turkish positions inside Turkey, Reuters reported.

“We will destroy the terror corridor gradually as we did in Jarablus and Al-Bab operations, starting from the west. The Afrin operation has de-facto started in the field. Manbij will follow,” Erdogan said, Reuters reported.

Turkey has in recent days sent dozens of military vehicles and hundreds of troops to the border area amid repeated threats from top officials that an operation could be launched at any moment.

Erdogan has repeatedly said he would crush the Syrian Kurdish militia in Afrin, which he views as a security threat to Turkey.

Direct military action against territory held by the Kurdish militia opens a new front in Syria’s war and sees Ankara confronting Kurds allied to the United States at a time when Turkey’s relations with Washington are reaching breaking point.

The US State Department called on Turkey to focus on the fight against Daesh terrorists and described Turkish activity in Afrin as destabilizing.

Erdogan had reacted furiously this week to an announcement of plans to create a US-backed 30,000-strong border security force in northern Syria composed partly of YPG militants, describing it as an “army of terror.”

AFP correspondents in the area around the Turkish border town of Reyhanli in Hatay Province saw several more Turkish military vehicles heading south to the border.

Syria’s Deputy Foreign Minister Faisal Mekdad warned on Thursday that the Syrian Air Force could destroy any Turkish warplanes used in a threatened assault on the war-torn country.